Event-based sensor that filters for flicker

Abstract

An event-based sensor includes a photoarray and a processing circuit. The photoarray includes an array of cells. Each cell includes a photosensor generating a sensor signal dependent on an intensity of light casted on the cell, and an intensity monitoring circuit outputting an ON signal when the light intensity is increasing and an OFF signal when the light intensity is decreasing. The processing circuit is configured to generate an event in response to the ON and OFF signals, filter out the events caused by flickering light, and pass the events caused by motion.

What is claimed is: 
     
       1. An event-based sensor comprising:
 a photoarray comprising an array of cells, where each cell includes a photosensor generating a sensor signal dependent on an intensity of light casted on the cell, and an intensity monitoring circuit outputting an ON signal when the light intensity is increasing and an OFF signal when the light intensity is decreasing; and 
 a processing circuit configured to generate a plurality of events in response to the ON and OFF signals, select one of the events having a polarity change, calculate a time difference from the selected event and a subsequent one of the events having a same polarity change, determine a flicker probability based on the time difference, and filter out the selected event caused by flickering light when the flicker probability is above a threshold probability. 
 
     
     
       2. The event-based sensor of  claim 1 , wherein each of the events indicate a time when the corresponding event occurred, the cell from which the corresponding event originated, and whether the light intensity applied to the cell is increasing or decreasing. 
     
     
       3. The event-based sensor of  claim 1 , wherein the flickering light is caused by light generated from a light source powered by an alternating current (AC) power grid. 
     
     
       4. The event-based sensor of  claim 1 , wherein the processing circuit filters out a current event among the events for one of the cells using a first probability distribution function (PDF) of time differences generated based on a scene having the flickering light and a second PDF of time differences generated based on the scene excluding the flickering light. 
     
     
       5. The event-based sensor of  claim 4 , wherein the scene having the flickering light excludes moving objects and the scene excluding the flickering light includes at least one moving object. 
     
     
       6. The event-based sensor of  claim 4 , wherein the processing circuit determines the time difference between the current event and a prior event of the events for the one cell having the same polarity change, determines the flicker probability based additionally on the first PDF and the second PDF, and filters out the current event when the flicker probability is above the threshold probability. 
     
     
       7. The event-based sensor of  claim 6 , wherein the processing circuit determines the flicker probability from a previous flicker probability for the one cell, a previous non-flicker probability for the one cell, a first emission probability of the time difference in the first PDF, and a second emission probability of the time difference in the second PDF. 
     
     
       8. The event-based sensor of  claim 7 , wherein the flicker probability=(the first emission probability*the previous flicker probability)/((the second emission probability*the previous non-flicker probability)+(the first emission probability*the previous flicker probability)). 
     
     
       9. A method of controlling an event-based sensor to filter out events caused by flickering light, the events indicating whether a cell of a photoarray of the event-based sensor receives light of an increasing or decreasing intensity, the method comprising:
 comparing, by a processing circuit, a current event among the events and a previous event among the events to determine whether a polarity change is occurring; 
 computing, by the processing circuit, a time difference between the current event and a prior event among the events when the polarity change occurs and the prior event has a same polarity change; 
 determining, by the processing circuit, a flicker probability using the time difference; and 
 filtering, by the processing circuit, out the current event when the flicker probability is above a threshold probability. 
 
     
     
       10. The method of  claim 9 , wherein the flickering light is caused by light generated from a light source powered by an alternating current (AC) power grid. 
     
     
       11. The method of  claim 9 , wherein the determining of the flicker probability is performed using a first probability distribution function (PDF) of time differences generated based on a scene having the flickering light and a second PDF of time differences generated based on a scene excluding the flickering light. 
     
     
       12. The method of  claim 11 , wherein the scene having the flickering light excludes moving objects and the scene excluding the flickering light includes at least one moving object. 
     
     
       13. The method of  claim 11 , the determining of the flicker probability comprises:
 determining the time difference between the current event and the prior event of the events for the cell having the same polarity change; and 
 setting the flicker probability based on the time difference, the first PDF, and the second PDF. 
 
     
     
       14. The method of  claim 13 , wherein the setting the flicker probability comprises setting the flicker probability based on a probability of the cell transitioning from a flicker state to a non-flicker state, a probability of the cell transitioning from the non-flicker state to the flicker state, a probability of maintaining the flicker state, and a probability of maintaining the non-flicker state. 
     
     
       15. The method of  claim 14 , wherein setting the flicker probability comprises setting the flicker probability based on a previous flicker probability for the cell, a previous non-flicker probability for the cell, a first emission probability of the time difference in the first PDF, and a second emission probability of the time difference in the second PDF. 
     
     
       16. The method of  claim 15 , wherein the flicker probability=(the probability of the cell transitioning from the non-flicker state to the flicker state*the previous non-flicker probability for the cell)+(the probability of maintaining the flicker state*the previous flicker probability for the cell). 
     
     
       17. The method of  claim 15 , wherein the flicker probability=(the first emission probability*the previous flicker probability for the cell)/((the second emission probability*the previous non-flicker probability for the cell)+(the first emission probability*the previous flicker probability for the cell)). 
     
     
       18. A method of controlling an event-based sensor including a photoarray that outputs signals for each cell of the photoarray indicating whether light is increasing or decreasing in intensity, the method comprising:
 generating, by a processing circuit of the event-based sensor, polarity events from each of the signals; 
 selecting, by the processing circuit, one of the polarity events having a polarity change; 
 calculating, by the processing circuit, a time difference from the selected polarity event and a subsequent one of the polarity events having a same polarity change; 
 determining, by the processing circuit, a flicker probability based on the time difference; and 
 filtering, by the processing circuit, out the selected polarity event if the flicker probability is above a threshold probability. 
 
     
     
       19. The method of  claim 18 , wherein the determining of the flicker probability is based on the time difference, a first probability distribution function (PDF) of time differences generated from a scene including flickering light, and a second PDF of time differences generated from the scene excluding the flickering light.